The present invention relates to a springing system for a vehicle having a hydraulic actuator and a storage element which holds hydraulic fluid, is constructed as a gas accumulator with a holding chamber for the hydraulic fluid and a separate gas chamber, and communicates with the actuator via a connecting line.
EP 0 427 046 A1 discloses a hydropneumatic springing system for a vehicle, which comprises a hydraulic setting cylinder between a wheel suspension and the structure of the vehicle in each case, the cylinder communicating with a gas accumulator. The two pressure chambers on either side of the piston of the setting cylinder are connected to the hydraulic fluid holding chamber of the gas accumulator. Thereby, during a movement of the piston of the setting cylinder, the hydraulic fluid is forced into the holding chamber of the gas accumulator or conveyed out of this. The holding chamber of the gas accumulator is separated from a gas chamber in the gas accumulator via a diaphragm, the compressible gas that is acted on by the hydraulic fluid via the diaphragm effecting a soft and elastic spring behavior in accordance with the gas characteristic curve; with increasing loadings, the gas characteristic curve runs progressively.
This hydropneumatic springing system has the disadvantage that it is not possible to relieve the load completely on the pneumatic gas accumulator. The gas accumulator is prestressed with a predefined pressure, and if the pressure falls below the prestress, the backing force drops off abruptly and force transmission is no longer possible. The result of this is that when a wheel is relieved of load, for example when travelling around curves, there is the risk that the unloaded spring strut will come into the range of an impermissibly low pressure below the prestress.
With the aid of a control concept, in which by way of a pump/motor device, hydraulic fluid is deliberately fed to the more highly loaded spring struts, depending on the degree of loading, it is certainly possible to avoid hazardous and unstable driving states. For example, swaying movements, in particular swaying oscillations, with increasing loading of the wheel on the inside of the curve and on the outside of the curve are prevented. In the event of failure of the regulation, however, this system does not offer any adequately safe emergency running behavior.
U.S. Pat. No. 3,955,807 describes a hydraulic spring strut of a vehicle, whose hydraulic chamber is connected to a storage element in which an elastomer is arranged. The elastomer in the storage element separates a hydraulic chamber from a gas chamber. During a dipping movement of the spring strut, hydraulic medium is forced into the hydraulic chamber of the storage element, and at the same time the elastomer is displaced in the direction of the gas chamber. Because of the different compressibilities of the gas in the gas chamber and of the elastomer, and because of the series connection of gas chamber and elastomer, the result is a nonlinear characteristic curve determined by the specific properties of the gas and of the elastomer.
This design has the disadvantage that a linear, Hookeian spring behavior cannot be simulated, because of the nonlinear gas properties. In addition, complete relieving of the load on the pneumatic gas accumulator is not possible with this design either.
FR-A-23 62 017 shows a springing system having two parallel gas/hydraulic accumulators, which are connected via a connecting line, in which there is an adjustable valve. Because of the difference in size between the two gas/hydraulic accumulators, different characteristic curves can be activated as a function of the loading acting on the springing system. However, because of the gas-specific properties, it is still only possible to implement a nonlinear springing behavior. On the other hand, a linear springing behavior is not possible.
U.S. Pat. No. 4,896,702 shows a spring leg which is comparable with that from FR-A-23 62 017. Depending on the movement of the springing, in each case one of the two gas/hydraulic accumulators is acted on. The springing behavior is also nonlinear.
The invention is based on the problem of developing a hydropneumatic springing system for vehicles with a largely linear spring behavior up to complete relief of the load.
According to the invention, this problem has been solved by providing a further storage element which is connected to the actuator via a connecting line and is constructed as an elastomeric accumulator.
The use of the additional elastomeric storage element achieves an approximately linear characteristic curve of the springing system up to complete relief of the load. The elastomer has a linear spring behavior in accordance with Hooke""s law, the spring constant being adjustable through the selection of the elastomer, depending on the desired stiffness.
The elastomeric accumulator supplements the gas accumulator, in particular at low loading. In the case where the elastomeric accumulator and the gas accumulator are arranged in parallel, and both accumulators are connected to the setting element via a common hydraulic fluid connecting line, the spring behavior of the accumulators can be matched to each other in such a way that the work is distributed between accumulators. At low loadings, a back-pressure is built up in the elastomeric accumulator and, in particular in the case of swaying movements, contributes to stable driving behavior. Under high loads, the elastomer expediently rests on a supporting housing and makes no contribution to the springing, which is then carried out exclusively by the gas accumulator. Because of the soft spring behavior of the gas accumulator, in this case in particular low-frequency structure oscillations can be cushioned.
In this case, the gas accumulator is expediently connected to the actuator via its own connecting line and assumes the function of a balancing accumulator, in particular of a central balancing accumulator for a number of actuators in a vehicle.
The balancing accumulator and the elastomeric accumulator are acted on in a frequency-selective manner in the passive system, in that low-frequency oscillations are fed to the balancing accumulator and higher-frequency oscillations are fed to the elastomeric accumulator. The frequency selection is advantageously carried out via a pump/motor device, which is arranged in the flow path between the balancing accumulator and the actuator and, because of its inherent dynamics in passive operation, essentially transmits only low frequencies up to the inherent frequency to the balancing accumulator. These frequencies correspond approximately to the typical frequencies of the vertical structure oscillations. At the same time, damping is achieved by restricting the volume flow in the pump/motor device. Higher-frequency are no longer transmitted to the balancing accumulator, because of the inertia of the pump/motor device, but are transmitted only to the elastomeric accumulator.
Because of the linear spring behavior of the elastomer, the latter is particularly well suited to active springing systems, for example to prevent or to overcompensate for swaying oscillations when travelling around curves. Depending on the regulation concept, however, further system degrees of freedom, such as dipping, lifting movements, self-steering behavior or a load-independent basic level of the springing can be influenced.
The springing system is expediently regulated via the pump/motor device, it being possible for either speed regulation or volume flow regulation to be used. If an electric motor is used in the pump/motor device, its speed is regulated. On the other hand, the volume flow of the fluid stream delivered by the pump is regulated if the pump/motor device is driven by the internal combustion engine of the vehicle at an advantageously constant speed.
It is preferable for at least two actuators to be connected together, by the two actuators of one axle or the diagonally opposite actuators front left/rear right or front right/rear left being acted on by a common pump/motor device. For each overall system, comprising elastomer accumulators, actuators and a pump/motor device, a central gas accumulator can be provided as a balancing accumulator. This system is hydraulically autonomous, the pump/motor device also running permanently and actively influencing the springing behavior or, in emergency operation, acting as a frequency divider. In addition, the passively operated pump/motor device can also act as a generator, by the pump, which is expediently constructed as an internal-gear pump, being operated by the hydraulic flow and driving the electric motor.
In order to achieve damping of oscillations going beyond increasing the flow resistance, a restrictor is advantageously arranged in the connecting line between the elastomeric accumulator and/or the gas accumulator, it being possible for this restrictor to be constructed as a fixed restrictor or also variably adjustable.
The actuator is preferably a bellows, in particular a rolling bellows or a folding bellows. The bellows is distinguished by a particularly low-friction behavior, as a result of which the lowest possible proportion of energy is dissipated.